1. Field of the Invention
The present invention relates to optical semiconductors comprising an amorphous optical semiconductor and a microcrystaline optical semiconductor, and processes of producing them, and an optical semiconductor element using any one of them.
2. Related Art
Heretofore, amorphous chalcogenide compounds of selenium, tellurium and the like have widely been used as a photoelectric conversion substance in image pickup tubes, photo-detectors, photosensitive materials for electrophotography and the like as an amorphous optical semiconductor (Amorphous Semiconductor Basics published by Ohm incorp.). In recent years, hydrogenated amorphous silicon has been come to be used in solar cells, image sensors, thin film transistors, photosensitive materials for electrophotography and the like.
Amorphous chalcogenide compounds, however, are subject to crystallization due to their instability to heat, thus the conditions in which they can be used with ease are limited, and they also have faults such as they cannot control valence electrons. Further, while in a hydrogenated amorphous silicon it is possible to control valence electrons, and thence to realize an electric field effect at an interface and pn junction, and also to have heat resistance up to about 250.degree. C., however, there is the problem where photoconductivity is degraded on exposure to a high intensity light (Staebler, Wronski effect: see Applied Physics Handbook and the like) and thereby the efficiency of a solar cell is reduced during use. Moreover, while a band gap of hydrogenated amorphous silicon is about 1.7 eV, and the band gap can be adjusted to be smaller or larger by addition of Ge and C thereto, in order to effectively utilize solar light, its photoconductive characteristics are subject to deterioration even with a change of the order of 0.3 eV in the bandgap, and thereby it has had the problem that light in a broad range of wavelengths cannot effectively be utilized. Further, semiconductors made of these elements, including a microcrystal and crystal, are all of an indirect transition type, and thereby they cannot be used as materials of a light emitting device, so that their applications have been restricted.
Heretofore, amorphous materials of group III-V compound semiconductors have been used for film formation, by evaporation or sputtering of group III-V compound crystals, or by the reaction of a metal of group III in an atomic state with molecules or activated molecules including an element of group V (H. Reuter, H. Schmitt and M. Boffgen, Thin Solid Films, 254, 94 (1995)). Further, a crystalline film of group III-V compound crystals has been produced on a heated substrate, using an organometallic compound including a group III metal, and a compound including a group V element (organometallic CVD: MOCVD). When one of these methods is applied, an amorphous group III-V compound has been able to be obtained by setting the temperature of the substrate lower than between 600 to 1000.degree. C., when producing crystals.
However, amorphous group III-V compounds have not been able to function as photoelectric materials, because of problems such as carbon from the organic metal being left in the film, many defect levels arising in the film and the like. On the other hand, it has been known that defect level density in the bands of amorphous silicon is reduced by hydrogenation and valence electrons can be controlled.
Many investigations have been conducted into the effects of hydrogen on defects in crystalline group III-V compounds with the following findings: (1) crystalline dislocations are improved, Y. Okada, S. Ohta, H. Shimomura, A. Kawabata and M. Kawabe, J. J. Appl. Phys., 32, L1556 (1993); (2) defects at the interface with the surface oxide film are improved, Y. Chang, W. Widdra, S. I. Yi, J. Merz, W. E. Weinberg and E Hu, J. Vac. Sci. Tech. B12. 2605 (1994); (3) the n+-p junction interface is improved, S. Min, W. C. Choi, H. Y. Cho, M. Yamaguchi, Appl. Phys. Lett 64, 1280 (1994); (4) defects from lattice mismatching are improved, B. Chatterjee, S. A. Ringel, R. Sieg, R. Hoffman and I. Weinberg, Appl. Phys. Lett 65, 58 (1994); and (5) bond defects are passivated. From the aforementioned effects of hydrogen, it can be expected that because of the problems in changing from a crystalline state to an amorphous state, the same relationship between crystalline silicon and amorphous silicon will occur in group III-V compound semiconductors.
On the other hand, it has been known that, in a III-V compound semiconductor, dopants for use in pn control are passivated at the same time as being inactivated (S. J. Pearton, Material Sci. Forum, 148 to 149, (1994) 113 to 139). It has also been known that dopants made inactive due to passivation, are reactivated by annealing. In the same way, hydrogen passivates dangling bond defects, but in the case of an amorphous film, the composition of the elements of which the films constructed, and the content of hydrogen in the film as well as its bonding positions are important, in order to inactivate the pn control dopants together with the dangling bonds.
In the conventional method of producing amorphous group III-V compound semiconductors, crystals are used as the raw material and produced by sputtering in a system containing no hydrogen, (H. Reuter, H. Schmitt, M. B. Offgen, Thin Solid Films 254, 94 (1995).
Further, regarding amorphous group III-V compound semiconductor containing hydrogen, hydrogenated a-GaP photoconductivity, obtained by reactive evaporation with H.sub.2 has been reported (M. Onuki, T. Fujii and H. Kubota, J. Non-Cryst, Solids, 114, 792 (1989)), and hydrogenated a-GaAs photoconductivity has also been reported (V. Coscia, R. Murri, N. Pinto, L. Trojani, J. Non. Cryst. Soild, 194 (1996) 103). In these semiconductors, however, the contrast resistance ratio is small, only to the order of two digits, moreover, pn control, which is indispensable for semiconductor material in terms of practical use, cannot be effected.
Further, amorphous GaP containing hydrogen or microcrystalline GaN have been obtained by the plasma CVD method, which is a low temperature film formation method which uses an organometallic compound as the group III raw material but they either show no photoconductivity or else insulation properties (J, Knight, and R. A. Lujan, J. Appl. Phys., 42. 1291 (1978)). A further problem has been that hydrogenated amorphous GaAs produced by the plasma CVD method, only has a very small photoconductivity, of the order of 10%, and is a long way from reaching the point of practical use, and since an organometallic compound is used as the raw material of group III, removal of carbon from the film is difficult in the low temperature regions required for obtaining an amorphous state (Y. Segui, F. Carrere and A. Bui, Thin Solid Films, 92, 303, (1982)).
While, in Japanese Patent Application Laid-Open (JP-A) No. 6-295991, amorphous group III-V compound semiconductors produced by either the sputtering method, the CVD method, the molecular beam epitaxy method, or the plasma CVD method have been proposed as an anti-fuse memory material which has a lower melting point than Si, it has been said that GaN and AlN are not suitable. And in the publication, there is no description of their characteristics as optical semiconductors.
A technique has been disclosed, as described in Japanese Patent Application Laid-Open (JP-A) 2-192770, that a combination of InN which has absorption in the visible light region and a small bandgap and amorphous silicon, is used as the microcrystalline material of a group III-V compound semiconductor. However, InN has a bandgap of 1.9 eV, and therefore it is necessary that the bandgap be variable over a broad range so that light may be absorbed and emitted efficiently over the total visible range. On the other hand, crystal GaN has a bandgap of the order of 3.1 eV and so has absorption in the ultraviolet region. At present, while buffer layers are widely used for the production of GaN crystals, the structure of the film is not very clear, although the temperature for growth has been to the order of 600.degree. C. lower than that for crystal growth. In cases where film formation is effected at a temperature lower than that of crystal growth, a microcrystalline film is thought to be grown. However, when crystal growth is continued, growth is effected in the range of 800 to 1000.degree. C. This step has only been used for alleviating lattice mismatching with the substrate, and microcrystaline compounds have not been used as a single film.